A pneumatic brake booster, of the type comprising:                an axially movable control member for controlling this booster and intended to receive a control force applied by a user,        a pneumatic enclosure containing two chambers, called a rear chamber and a front chamber, separated by a pneumatic piston, and        means forming pneumatic valves comprising:                    first and second complementary valve-forming elements for placing the rear chamber in communication with the front chamber, called first and second balancing valve elements, and            first and second complementary valve-forming elements for placing the rear chamber in communication with a pneumatic pressure source, called first and second intake valve elements,                        
is already known from the prior art.
The control member, also referred to as a plunger, is usually intended to transmit a braking force to a master cylinder control rod. The pneumatic piston is connected to the control rod.
A braking operation is controlled by the driver by means of a brake pedal connected to the braking device control member.
The pressure in the master cylinder changes as a function of the force exerted on the brake pedal, called braking force, in the following way.
Initially, the braking force is intended to overcome the preload on a return spring which returns the control member to a rest position. The booster remains at rest. The pressure in the master cylinder remains constant.
As the braking force increases, the preload on the control member return spring is overcome and the booster is activated. Now, with the booster in its rest configuration, there is usually an axial clearance between the control member and the control rod. Consequently, activating the booster causes a pressure jump in the master cylinder, this occurring with a constant braking force, since, on account of the axial clearance, the control member is not subjected to any reaction on the part of the master cylinder control rod.
It will therefore be appreciated that the greater the pressure jump in the master cylinder, the more rapidly effective the action on the brake pedal will be.
After canceling the axial clearance, the pressure in the master cylinder increases proportionally to the braking force, first of all at a ratio corresponding to the boost ratio of the booster, and then, after boost saturation, at a ratio which is less than the preceding one.